West Nile virus (WNV) is a member of the Flavivirus genus of the Flaviviridae which also includes the Japanese encephalitis virus (JE), Tick-borne encephalitis virus (TBE), St. Louis Encephalitis virus (SLEV), Murray Valley encephalitis virus, dengue virus (including the four serotypes of: DEN-1, DEN-2, DEN-3, and DEN-4), and the family prototype, yellow fever virus (YF). Flavivirus infections are a global public health problem [C. G. Hayes, in The Arboviruses: Epidemiology and Ecology, T. P. Monathy, ed., CRC, Boca Raton, Fla., vol. 5, chap. 49 (1989); M. J. Cardosa, Br Med Bull, 54, pp, 395-405 (1998); Z. Hubalek and J. Halouzka, Emerg Infect Dis, 5, pp. 643-50 (1999)] with about half of the flaviviruses causing human diseases.
WNV is a neurotropic, enveloped virus with a single-stranded, positive polarity, 11 kilobase RNA genome. Until 1999, WNV was found in the Eastern Hemisphere, with wide distribution in Africa, Asia, the Middle East, and Europe (1). In 1999, WNV entered the Western Hemisphere as a point introduction in New York City (2), Greater than 29,000 human cases have been diagnosed with severe WNV infection in the continental United States during the last decade, and many more have been infected and remain undiagnosed. Advanced age is by far the greatest risk factor for severe neurological disease, long-term morbidity, and death (3), although a genetic basis of susceptibility has also been recently identified (4-7).
Historically, there has been a lack of effective and specific antiviral treatment for infection by WNV or other flaviviruses (reviewed in 8). While several small molecules compounds have been recently described with antiviral activity against WNV in vitro, only few have demonstrated efficacy in vivo (9, 10). Interferon (IFN), which is used as part of combination therapy against the distantly related, hepatitis C virus, potently inhibits flaviviruses including WNV when used as prophylaxis. However, its effect is markedly attenuated once viral replication has commenced as flavivirus non-structural proteins antagonize IFN signaling pathways (reviewed in 11). Current treatment for WNV infection is supportive and no vaccine or therapeutic agent has been approved for human use. New threats of WNV globally and lack of available treatments warrant studies to develop effective therapeutics and production technologies that can rapidly transfer the candidates into the clinical care settings in a cost-conscious manner.
Recently, a plant-derived humanized murine MAb was developed with promising therapeutic potential. This MAb (E16) binds to a highly conserved epitope on the envelope protein of WNV in all North American isolates, blocks viral fusion, and shows promising post-exposure therapeutic activity. Nonetheless, detailed studies show that while the E16 is therapeutically effective, peripheral delivery of this antibody has a limited window of efficacy in rodents. For example, administration of a single dose of hu-E16 through an intravenous or intraperitonreal route at day 5 postinfection or earlier improves survival rates. However, delivers of E.16 directly into the brain at day 6 after infection can protect hamsters against lethal WNV infection,
Thus, even though antibodies have been identified as potential prophylactic and or therapeutic medicaments for WNV or other infectious diseases, their ultimate application as beneficial therapeutics is limited lack of efficacy due to the short therapeutic window. Thus, there remains a need for blood-brain barrier permeable variants of antibodies that can achieve higher levels in the CNS, increase its therapeutic efficacy, and extend the window of treatment. In addition, the there is a need to alleviate the high production costs and scalability associated with the mammalian cell culture production system. Moreover, if biological drugs are too costly to produce for resource poor health care systems and cannot be easily made into generics, their therapeutic potential may never be realized. As such, the development of production platforms that are cost-effective, scalable, and safe for biological therapeutics is urgently needed.